U.S. Pat. No. 6,014,006 discloses a method for controlling torque output for an electric induction motor using estimations of rotor flux angle and rotor speed. The speed is estimated based on motor dynamics represented by equations for which the speed is a control variable to be estimated, the other variables being measurable. Rotor flux is estimated directly from rotor speed using a set of motor control equations, and rotor flux angle is determined from rotor flux for controlling motor torque.
The accuracy of speed estimation of an induction motor without rotational transducers can be affected by rotor resistance variation during operation.
A rotor parameter identification technique for the purpose of updating control gains of an induction motor vector controller by introducing a separate high frequency carrier signal is described by Matsuo et al in a paper entitled "A Rotor Parameter Identification Scheme For Vector-Controlled Induction Motor Drives", (IEEE Transactions On Industry Applications, Vol. IA-21, No. 4, May/June 1985). An adaptive scheme for simultaneous speed and rotor resistance identification derived by using Lyapunov's function is analyzed by Kubota et al in a paper entitled "Speed Sensorless Field-Oriented Control of Induction Motor with Rotor Resistance Adaptation" (IEEE Transactions on Industry Applications, Vol. 30, No. 5, 1994). The approach of Kubota et al is implemented by superimposing AC components on the field current command.
Robust nonlinear control of the speed of induction motors that uses measurements of the rotor position and stator current with on-line adaptation of the rotor resistance is described by Aloliwi et al in a paper entitled "Robust Speed Control of Induction Motors Using Adaptive Observer" (Proceedings of American Control Conference, pp. 931-935, June 1999). A paper written by Shishkin et al entitled "Discrete-Time Method for Robust Global Stabilization of Induction Motor" (Proceedings of American Control Conference, 1999), describes how the rotor resistance is updated at a prescribed instant by minimizing a nonlinear scalar function, which is monotonic and has a well defined solution when .omega. and/or load are small.
Zheng et al describe in a paper entitled "Adaptive Flux and Speed Estimation for Induction Motors" (Proceedings of American Control Conference, June 1999), an adaptive estimator for rotor resistance that considers the rotor speed and rotor resistance as slowly varying unknown parameters of a model reference adaptive system. Under persistent excitation, it is explained by Zheng et al that estimated flux and rotor speed converge to their true values.
A paper by Bondarko and Zaremba (one of the inventors of the present invention), entitled "Speed and Flux Estimation for an Induction Motor Without Position Sensor" (Proceedings of American Control Conference, June 1999) describes a speed sensorless torque control of an induction motor.